This invention relates in general to microwave signal frequency measurement devices, and more particularly to a digital instantaneous frequency measurement receiver utilizing cascaded analog frequency dividers.
Various prior art systems have been developed for use in electronic warfare, for automatically measuring and displaying the frequency of microwave signals and thereby effect electronic support measures, electronic counter measures and electronic intelligence applications.
One prior art approach to microwave frequency measurements utilized a heterodyne converter frequency counter for mixing a predetermined harmonic of a local oscillator signal with an unknown frequency microwave input signal, resulting in sum and difference frequency signals. The heterodyne converter frequency counter measured the resulting difference frequency which was then counted directly by a digital counter. The exact frequency of the local oscillator and the value of the harmonic are both known, such that the resulting difference frequency forms a representation of the detected microwave signal frequency offset by a predetermined scaling factor proportional to the value of the harmonic.
Another well known prior art technique utilized a transfer oscillator which was tuned to a frequency for which the difference frequency between the unknown input signal and the predetermined harmonic of the local oscillator was reduced to zero. The frequency of the local oscillator was then directly measured by a digital counter and multiplied by a scaling factor corresponding to the value of the harmonic, and the resulting measured input signal frequency was then displayed.
It has been discovered that heterodyne conversion frequency counters are limited in their ability to measure the frequency of pulsed microwave signals, and while transfer oscillator frequency counters are capable of measuring pulsed microwave signals, they are typically incapable of measuring signals with large frequency modulation components.
In an effort to overcome the disadvantages of the aforementioned prior art systems, another prior art system was developed wherein a plurlaity of analog microwave frequency dividers were utilized to down-convert the received microwave signal in octave steps into the frequency regions associated with digital frequency counters. The resulting frequency displayed by the frequency counter represented a measure of the input signal frequency reduced by a factor of 2.sup.n, where "n" corresponds to the number of cascaded frequency divider stages utilized.
This prior art system is described in Canadian Pat. No. 1,085,925 issued Sept. 16, 1980 and entitled Apparatus for Measuring the Frequency of Microwave Signals.
According to a successful implementation of the prior art system, a frequency divider chain was utilized to compress an input microwave signal frequency bandwidth of from 8 to 12 gigahertz down to a bandwidth of from 1 to 1.5 gigahertz. The down-converted signal was then detected by a gate control circuit for enabling a well known digital counter circuit, such as a GaAs counter, for a fixed time interval in response to the received down-converted signal having an amplitude in excess of a predetermined threshold. A time delay circuit was connected to the counter input to ensure that the down-converted signal was applied to the input of the counter simultaneously with the counter being enabled by the gate control circuit. The counter accumulated received microwave signal pulses during the aforementioned fixed time interval resulting in a measure of the down-converted input signal frequency represented symbolically by F=N/T, where "F" is the radio input signal frequency in gigahertz, "N" is the counter output value, and "T" is the fixed time interval in nanoseconds. The resulting measured frequency was then multiplied by the aforementioned factor of 2.sup.n, to arrive at a measure of the input microwave signal frequency.
For typical electronic warfare applications, the input microwave signal pulse widths are typically too narrow (eg. the minimum pulse width is typically less than 100 nanoseconds) for accurate measurement using standard digital counters, resulting in inaccurate frequency measurement.
The following table lists a measure of the frequency accuracy (in megahertz), for three radio frequency bandwidths of 8-12, 4-8 and 2-4 GHz for input signal pulse widths of 0.1, 1 and 10 microseconds.
TABLE 1 ______________________________________ FREQUENCY (GHz) 8-12 4-8 2-4 ______________________________________ PW 0.1 .+-.80 .+-.80 .+-.40 (.mu.S) 1.0 .+-.8 .+-.8 .+-.4 10.0 .+-.0.8 .+-.0.8 .+-.0.4 FREQUENCY ACCURACY (MHz) ______________________________________
As seen from Table 1, for typical electronic warfare applications in which the input microwave signal pulse widths are less than 100 nanoseconds (i.e., 0.1 microseconds), the frequency accuracy is very low, (i.e., on the order of .+-.80 MHz for input microwave signals having frequencies from 4-12 GHz, and .+-.40 MHz for input microwave signal frequencies in the range of from 2-4 GHz).
One attempt to overcome the prior art frequency accuracy difficulties was to accumulate successive frequency measurements over several pulses, and then average the results. However, for typical electronic warfare applications wherein the pulse density is greater than 10 MPPS, this alternative is not frequently practical due to frequency de-interleaving of the multiple input microwave signals, and long throughput times (eg. 10 ms) for implementing the accumulation and averaging process.